1. Field of the Invention
The present invention relates to a semiconductor device for wireless communication. The present invention particularly relates to a semiconductor device for supplying a power source voltage which is generated from a communication signal, to a circuit formed by a semiconductor thin film transistor.
2. Description of the Related Art
In recent years, a compact semiconductor device in which a microminiaturized IC chip is combined with an antenna for wireless communication (this semiconductor device is also called a wireless chip hereinafter) is attracting attention. By sending and receiving a communication signal with the use of a wireless communication device (hereinafter referred to as a reader/writer), data can be written in or read from a wireless chip.
As an application field of wireless chips, for example, merchandise management in the distribution industry is known. A merchandise management system using a bar code is the mainstream at present; however, since bar codes are read optically, data cannot be read when there is an interrupting object. On the other hand, when wireless chips are used for merchandise management, data is read wirelessly. Therefore, data can be read even though there is an interrupting object, as long as a wireless communication signal can transmit through the interrupting object. Thus, improvement in efficiency, cost reduction, and the like of merchandise management are expected by using wireless chips for merchandise management. In addition, wide range of applications including boarding passes, airplane tickets, automatic payment of fares, etc. are expected (see Reference 1: Japanese Published Patent Application No.: 2000-149194).
Description is made of a method for generating a power source voltage from a communication signal in a wireless chip, with reference to FIG. 3 and FIGS. 4A and 4B. FIG. 3 shows a power source circuit in a wireless chip, and FIGS. 4A and 4B show change of a voltage over time in each part of the power source circuit.
In FIG. 3, the power source circuit has an antenna portion 301, a rectifying portion 302, and a storage capacitor portion 303. The antenna portion 301 has an antenna 304 and a resonance capacitor 305. By receiving a communication signal, a potential difference is generated between a first output terminal 306 and a second output terminal 307 of the antenna portion 301 (hereinafter this potential difference is called an output potential of the antenna portion 301). The rectifying portion 302 includes a diode 308. For simple description, the rectifying portion 302 is regarded as a half-wave rectifying portion. The first output terminal 306 and the second output terminal 307 of the antenna portion 301 are connected to a first input terminal 309 and a second input terminal 310 of the rectifying portion 302, and a rectified potential difference is generated between a first output terminal 311 and a second output terminal 312 of the rectifying portion 302 (hereinafter this potential difference is called an output potential of the rectifying portion 302). The storage capacitor portion 303 has a storage capacitor 313. The first output terminal 311 and the second output terminal 312 of the rectifying portion 302 are connected to the first input terminal 314 and the second input terminal 315 of the storage capacitor portion 303, and a potential difference is generated between the first output terminal 316 and the second output terminal 317 of the storage capacitor portion 303 (hereinafter this potential difference is called an output potential of the storage capacitor portion 303). The output potential of the storage capacitor portion 303 is a power source voltage of a wireless chip.
Change of the output potential over time in the antenna portion 301 of FIG. 3 is shown by a waveform 401 of FIG. 4A. At this time, change of the output potential over time in the rectifying portion 302 of FIG. 3 is shown by a waveform 402 of FIG. 4B. Moreover, change of the output potential over time in the storage capacitor portion 303 of FIG. 3 is shown by a waveform 403 of FIG. 4B. The diode 308 in the rectifying portion 302 is conductive only if the first input terminal 309 has a higher potential than the first output terminal 311 in the rectifying portion 302. Thus, the diode 308 has a function of rectifying current only in a portion where the output potential of the antenna portion 301 is positive. Although the output potential of the rectifying portion 302 is smoothed by the storage capacitor 313 to be the output potential of the storage capacitor portion 303, the output potential of the storage capacitor portion 303 gradually decreases because it is consumed as electric power by circuits of a wireless chip. Since the aforementioned step is repeated, the output potential of the storage capacitor portion 303 changes over time as the waveform 403.
As set forth above, since a power source voltage is generated from a communication signal in a wireless chip, there is a risk that a large amount of voltage that could electrically destroy a circuit be generated in the wireless chip in the case of supplying a communication signal with strong amplitude. With this risk in mind, a third party may supply a communication signal with strong amplitude on purpose to electrically destroy a circuit so that the information cannot be read from the wireless chip. Destruction of a wireless chip by such a communication signal with strong amplitude is hereinafter called a strong radiowave attack. In order to avoid the situation that information cannot be read from a wireless chip, a wireless chip needs to resist such a strong radiowave attack.